The present invention relates generally to an overhead projection system and, more specifically, to a method and system for compensating for the keystoning and focus distortion effects of an image projected obliquely onto a vertical screen.
Conventional overhead projectors include a horizontal platen which supports a slide transparency to be projected with a lamp beneath the platen to illuminate the transparency. A projection lens and mirror are mounted above the transparency and direct the light image of the transparency onto a viewing screen. To provide the audience with an unobstructed view, the mirror is commonly tilted to an angle greater than 45.degree. with respect to the horizontal, thereby raising the projected image above the level of the projector. If the viewing screen remains vertical, this projection of the image at an oblique angle above the horizontal results in a magnification distortion of the image into a trapezoidal shape in which the upper portion of the image is wider than the lower portion. Because of the shape of the projected image, this magnification distortion is commonly referred to as "keystoning".
The phenomenon is illustrated with reference to FIG. 1 which shows a side view of an overhead projection system. A projector 2 is placed on a table 4 at a height of, for example, about 3 feet above the floor. Projector 2 includes a platen 6 upon which is placed a transparency 8 to be projected onto a screen 10. The light image of the transparency is projected by lens 12 and reflected by fold mirror 14 along optical axis 16. The center of screen 10 is positioned at a height, say 6 feet, to maximize the visibility of the projected image. Mirror 14 is adjusted by the user to center the image of the slide transparency on the screen. When the center of the projection screen is higher than the height of the fold mirror, the optical axis is tilted upward from the horizontal, as shown in FIG. 1. Since the projection is vertical, the optical axis is not perpendicular to the projection screen. This non-perpendicularity results in a distorted image 8' on the screen as shown in FIG. 2, a horizontal line H at the top of the image will appear longer, on the screen, than a horizontal line H' of equal length (on the transparency), at the bottom of the slide. Parallel vertical lines V, V' will appear to be further apart at the top of the screen than at the bottom of the screen. There will also be focus differences between the top of the screen and the bottom of the screen, although using a projector lens 12 with ample depth of field will eliminate this problem.
Various prior art techniques are known to correct for this keystoning or projected image distortion. U.S. Pat. No. 4,089,599 proposes adding a correcting positive or negative lens between the transparency and the projection lens and adjusting the inclination of the correcting lens to compensate for the obliqueness of the projected image. U.S. Pat. No. 5,220,363 describes the same technique in the context of an enlargement projection apparatus. U.S. Pat. No. 4,436,393 describes a projection system where the projection lens and the fold mirrors are moved from a central position on the optical axis towards the screen while simultaneously diverting the light through the transparency at an oblique angle by offsetting the upper lens of a two-element fresnel condensing lens system. In U.S. Pat. No. 4,863,263 the projection lens and mirror are pivotably movable with respect to each other to adjust for the distortion.
The above prior art describes correction techniques which require the addition of optical components or some rearrangement of the lens and mirror of the projector to correct for keystoning and focus distortion. It would be desirable to correct for these distortions without the requirement for altering the conventional construction and/or operation of an overhead projector.
According to the invention, computer software is used to create a print or transparency which is intentionally distorted, or computer hardware is used to generate a distorted image on a flat panel display. For purposes of description, the term "transparency" will heretofore be used to designate either a slide or a projection panel display. The distortion is intended primarily to compensate for the keystoning introduced in the projection system, but it can also compensate for barrel and pin-cushion distortions. Focus distortions cannot be corrected by intentionally distorting the image, but, according to another aspect of the invention, a focus correction is made by slanting the intentionally distorted image.
In one embodiment of the invention, a test slide, shown in FIG. 3 is projected on the screen where an oblique angle of projection is likely to generate keystone distortion. The viewer chooses which pair of lines looks parallel, and thereby identifies a correcting distortion. Next all slides are printed with that same keystone or projective transformation. An example of a distorted slide 20' is shown in FIG. 4. Alternatively, slides may be printed in advance under a variety of distortions to match the lines on the test template. Finally, when the slide is projected, the contents of the slide appear with little or no distortion on the screen.
More particularly, the present invention relates to a method for correcting for distortion of an image projected by an overhead projector at an oblique angle above a horizontal plane onto a projection screen comprising the steps of:
creating a transparency of an intentionally distorted image, the distortion compensating for the keystoning caused by the oblique angle of projection, and PA1 illuminating the transparency and projecting the illuminated image onto said projection screen to form an undistorted image thereon.